CostModel: add another known vector trunc optimization.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167488 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index 5853b51..bdbc9bb 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -17692,6 +17692,7 @@
     { ISD::FP_TO_SINT,  MVT::v4i8, MVT::v4f32,  1 },
     { ISD::ZERO_EXTEND, MVT::v8i32, MVT::v8i1,  6 },
     { ISD::SIGN_EXTEND, MVT::v8i32, MVT::v8i1,  9 },
+    { ISD::TRUNCATE,    MVT::v8i32, MVT::v8i64, 3 },
   };
 
   if (ST.hasAVX()) {